Parasitic protozoa are responsible for a wide variety of infections in man and animals. Many of the diseases are life threatening to the host and cause considerable economic loss in animal husbandry. For example, malaria remains a significant health threat to humans despite massive international attempts to eradicate the disease; trypanosomiasis such as Chagas disease caused by Trypanosoma cruzi and African sleeping sickness caused by T. brucei are not uncommon in Africa and South America; and opportunistic infections in immunocompromised hosts caused by Pneumocystis carinii, Toxoplasma gondii, Cryptosporidium sp. are becoming increasingly significant in the developed countries.
A protozoal infection of great economic importance is coccidiosis, a widespread disease of domesticated animals produced by infections by protozoa of the genus Eimeria. Some of the most significant of Eimeria species are those in poultry namely E. tenella, E. acervulina, E. necatrix, E. brunetti and E. maxima. The disease is responsible for high levels of morbidity and mortality in poultry and can result in extreme economic losses.
In some protozoal diseases, such as Chagas disease, there is no satisfactory treatment; in others, drug-resistant strains of the protozoa may develop. Accordingly, there exists a continued need to identify new and effective anti-protozoal drugs. However, antiparasitic drug discovery has been, for the most part, a random and laborious process through biological screening of natural products and synthetic compounds against a panel of parasites. This process can be greatly facilitated and made more specific if a target of antiprotozoal drugs can be identified, and incorporated into the screening process.
Histone deacetylase and histone acetyltransferase together control the net level of acetylation of histones. Inhibition of the action of histone deacetylase results in the accumulation of hyperacetylated histones, which in turn is implicated in a variety of cellular responses, including altered gene expression, cell differentiation and cell-cycle arrest. Recently, trichostatin A and trapoxin A have been reported as reversible and irreversible inhibitors, respectively, of mammalian histone deacetylase (see e.g., Yoshida et al., BioEssays, 1995, 17(5):423-430). Trichostatin A has also been reported to inhibit partially purified yeast histone deacetylase (Sanchez del Pino et al., Biochem. J., 1994, 303:723-729). Trichostatin A is an antifungal antibiotic and has been shown to have anti-trichomonal activity as well as cell differentiating activity in murine erythroleukemia cells, and the ability to induce phenotypic reversion in sis-transformed fibroblast cells (see e.g. U.S. Pat. No. 4,218,478; Yoshida et al., BioEssays, 1995, 17(5):423-430 and references cited therein). Trapoxin A, a cyclic tetrapeptide, induces morphological reversion of v-sis-transformed NIH3T3 cells (Yoshida and Sugita, Jap. J. Cancer Res., 1992, 83(4):324-328). The present inventors have found that a number of cyclic tetrapeptides structurally related to trapoxin A are inhibitors of histone deacetylase and also possess antiprotozoal activity.